Beam-Down Demos First Direct Solar Storage at 1/2 MWh Scale
Typically, Power Tower CSP heats molten salts by shining concentrated sunlight on a receiver on top of a tower where molten salts flow through pipes in the receiver to be heated and sent down the tower to use the solar energy as heat in a hot tank below to save.
But what if the molten salt in the thermal store could simply be heated directly by concentrated solar power?
A 2021 paper Dispatchable solar power using molten salt irradiated directly from above describes the first attempt at such an outdoor facility at a magnitude of 600 kWh at the Khalifa University Masdar Institute solar platform test site. There is a bath of molten salts on the floor, and concentrated sunlight is shone on an inverted mirror above the tank and then reflected into the molten salt bath on the floor.
This reverse design, combining collection and storage, has many thermal efficiency benefits. It is much more convenient to install, maintain, and operate because the receiver is not at the top of a 140-meter tower; it’s on the ground.
“You save the entire pump energy. You don’t have to put the salt on top of a tower because instead of moving the salt, you move the light, ”commented Dr. Nicolas Calvet, lead author of the study.
Since the heat-absorbing material is the deep vat with clear liquid, there is also no risk of a hot spot, such as a tower receiver, being focused on the metal receiver tubes by too much solar power.
“In the tower you also have more constraints during the starting process. You have to empty the container every night so that the salt doesn’t crystallize when it cools in the pipe overnight, ”he explained. This adds additional shutdown time in the evening and start time in the morning.
“But at night, in our case, we just close the tank with a lid and that’s it. And in the morning you simply open the tank and concentrate the full light without taking any precautions, because there is no thermal shock from metal cooling. “
The ground orientation enables a further thermal efficiency measure. In many CSP laboratories that test solar thermochemical reactors, a quartz window closes the reactor to reduce heat loss, but the heat can break the glass from dust in hot spots, according to Calvet. But to keep the heat inside, since the tank of molten salt is on the bottom, the team could instead simply propel clear glass spheres onto the surface to insulate them.
In order to reduce the heat losses through radiation even further, an optical cone-shaped funnel concentrates the sunlight that has already been reflected twice as it enters the tank and reflects the heat radiation back into the liquid so that the opening is smaller than the tank diameter.
“You lose maybe 10% of the optical efficiency down the beam because every time you add a layer of mirror you lose some optical efficiency,” noted Calvet. “So we lose in terms of optical efficiency, but because of our direct absorption, we gain in terms of thermal efficiency.”
Thermocline storage +
Another “first” of these published experiments is a test of thermocline storage with a partition plate. Thermocline storage, in which the hot liquid is above the cold in a single tank, is increasingly being explored as a cost-cutting measure in CSP as it lowers costs. In the conventional tower you have the cold salt tank and the hot salt tank and yet only one tank volume is really used, since the molten salt always flows in and out between the two tanks, as it is heated and cooled by the sun as soon as its heat is withdrawn.
However, if you store both temperatures in a single tank, you need to reduce the thermocline, the area of mixed temperatures in the middle, as much as possible. Calvet’s team adapted a separator idea tested by SENER (but only for heat storage) in order to keep the two temperatures physically separate.
A metal divider would start at the top of the tank in the morning and sink down all day as the sun heats up from top to bottom; while underneath the used cold layer, which is returned from the power block, is warmed up again by getting into the gap between the wall side and the end of the partition plate and mixing with the hot salt.
(However, the partition plate is not required if solar power is only provided at night, as the entire volume would be heated up in eight hours of sunshine in order to be ready for use when it gets dark.)
The team also found a mixing plate useful, which homogenizes the hot layer on the divider. This recirculates the heated part at the top while increasing its volume to reach the molten salt at a deeper depth.
“When we started this experiment, Beam Down had very bad press,” he admitted. “People didn’t even think that what we did was a technology that would last in the future. Today we see a 50 megawatt commercial jet at Yumen in China; the first commercial beam down. We see research institutes like CSIRO in Australia converting their tower into Beam Down. We’re seeing more and more interest in beam-down technology. “
He and his colleague Professor Peter Armstrong came up with the idea of testing Beam Down for directly irradiated storage when he was employed as a professor in the Faculty of Mechanical Engineering at the Masdar Institute (MI, later merged with Khalifa University) in the United Arab Emirates became .
There he founded the Masdar Institute Solar Platform (MISP), which already had a half-abandoned test beam down tower that had been built by Masdar and a Japanese consortium in 2009, along with a very small 33-heliostat solar field – 280 square meters Mirrors.
“So I started looking for things that we could do with a beam down application. These are very interesting because, for example, you could think of melting metals with the sun to replace electricity and fuels. That was our original idea because you can imagine a tank at the bottom with some liquid in the tank and the light coming from above. “
Ultimately, Armstrong and Calvet decided to use the downward beam to test direct heating of the volume of the heat accumulator after Armstrong recommended a paper by Professor Alexander Slocum at MIT; Concentrated solar power on demand that has successfully tested direct absorption by solar receivers / storage with a solar simulator, but on a very small scale with only 7 liters of molten salt.
So Calvet contacted Slocum and proposed the MIT / MI joint collaboration to increase the idea from seven liters to four tons of molten salt using the existing beam down setup and this became the CSPond demo, one of the MIT / MI flagship programs.
The old test solar field of only 33 heliostats from 2009, however, consisted of what he described as a “damaged bathroom mirror”.
“Imagine putting a bathroom mirror with UV and this corrosive environment in the desert! They were dismantled in 2013; You should dismantle it in 2011. I think the Japanese wanted to save money on the project, ”he said.
New commercial implications
Even with the poor quality solar field, the team demonstrated that it reached 500 ° C. The smelting salt tower CSP is operated thermally at 565 ° C, which is sufficient for generating electricity in today’s power plants.
Calvet has now been able to order state-of-the-art Heliostat mirrors from Rioglass and is confident that, with a real solar field, the molten salts will reach the temperature required for the efficient operation of a power plant unit.
Conventional molten nitrate salts have a limit of 600 ° C to avoid decomposition, but Calvet found that, like tower CSP, much higher temperatures could be reached with new heat storage materials.
Some high temperature storage materials currently being investigated in ground level tanks for beam-down with direct storage include rocks and hot air or particles such as desert sand. In high-temperature industrial processes, it is advantageous to work on the ground.
These thermal storage materials for higher temperatures are currently being tested and can be operated at up to 1250 ° C for state-of-the-art thermochemical solar processes such as the production of solar fuels such as hydrogen and aviation fuel.
“But next we need a demonstrator. We are in “Death Valley”; Calvet commented.
“We have a proof of concept here, but it’s very small; 600 kWh electric; 100 kWh. We’d have to build a two to three megawatt electric demonstrator that would cost several million, maybe five million dollars. So we need someone to invest five million dollars in a large system to be economically viable. “
But this role in a startup would not fall to him: “I’m a professor. I am not a business developer. So my role is to show that it works; what we have now done with this proof of concept. “